<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
        /* 声明提前，变量提升 
        javascript在执行代码的时候会将所有使用var声明的变量，放在作用域的顶部集中创建，赋值留在原地
        */
        console.log(a);
        var a=20;
        function go(){
            console.log("hello world")
        }
        var b="good"
    </script>
</body>
</html>